Dust in air directly taken into the cylinder of internal combustion engines accelerates abrasion of the cylinder and piston and contaminates the engine oil which causes the bearings to be quickly abraded. Therefore, the air cleaner is installed on the internal combustion engine to remove dust from air being fed to the engine. The air cleaner also has the functions of improving engine output, decreasing pulsatory noises due to operation of the intake throttle valve and air suction noises, including air current noises due to the turbulence of air flow, and controlling fuel supply by dampening the pulsating flow of the suction air.
The structure of an air cleaner having the above functions is disclosed in Japanese application No. 61-48966. This air cleaner is designed so that the cross section of the air-incoming chamber formed between the pre-filter element and the bottom wall of the bottom case gradually decreases from the entrance.
FIG. 3 shows the structure of another known air cleaner. The air cleaner 4 shown in FIG. 3 has a case 10 consisting of a bottom portion 6 having a suction air passage 20 formed at an intake port 18, and a top portion 8 having an outgoing air passage 32 for supplying clean air to an air feed pipe 34 connected to the internal combustion engine 2. A filter element 12 is installed in the case 10. The suction air passage 20 of the intake port 18 is also connected to an intake pipe 28 which forms an intake passage 26 for taking in air from the outside.
In the structure of existing air cleaners, however, air suction noises produced at the downstream side of the air cleaner 4 easily pass through the suction air passage 20 and are propagated to the intake passage 26 side (shown by the arrow R in FIG. 3) because the passage cross section D of the suction air passage 20 is the same for the upstream-side opening 22 and the downstream-side opening 24. Therefore, there are the disadvantages that noises increase inside and outside a vehicle and the flow of suction air pulsates.